Finite-size effect on vortex loop excitation in amorphous-W/Si multilayers

Abstract

A finite size effect on vortex configuration is investigated by the measurements of current–voltage ( I– V) characteristics on amorphous-W/Si multilayers as a function of the number of the bilayers N in the absence of field. Critical scaling analysis of the data for the samples with N≥10 shows that the vortex loops are the favored excitation with the dynamic critical exponent z∼3.32, while for 10> N≥3 it reveals to deviate from the scaling function corresponding with a finite size effect on vortex loops. At a small current density, the vortex loop is probed at a length scale larger than the total sample thickness. A remarkable change in the I– V characteristics is observed between N=3 and 2. This indicates the vortex configuration crossover to vortex–antivortex string pairs piercing all layers at N=2. In sample with N=1, the typical KT transition is observed.